I'm just going to toss out one of the better deals (although you would have to go at good lengths to do it). I'm assuming your running an Intel P4C in order to use these high-speed memory.
Right now, the best buy is purchasing 2x256MB ADATA PC4000 from either Japan or China for around $65-70 a piece. Estimate around $150-$165 for it shipped to US, although you would be best to have someone you know who speaks Japanese or Chinese, or who is over there now, to buy it for you.
2x256MB Duel-Channel GeIL Ultra-Platinum PC4000 costs about $155.